Automatic gain and noise limiting circuit for a radar range unit



c. D. cocKBuRN 2,852,672 AUTOMATICVGAIN AND NOISE LIMITING CIRCUIT Foa A RADAR RANGE UNIT Filed Aug. s1. 195s y sept. 1 6, 1958 Patented Sept. 16, 1958 AUTMATIC GAIN AND NOISE LIMITING CIR- CUIT FUR A RADAR RANGE UNIT Curtis D. Cockburn, Baldvvinsvilie, N. Y., assigner, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application August Vfil, 1953, Serial No. 377,750

2 Claims. (Cl. Z50-27) The invention described herein may be manufactured by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to apparatus for automatically controlling the response of an electromagnetic sensitive device or the like and more particularly to novel and improved automatic gain control and noise limiter circuits which are specially designed for use in an electronic radar range unit.

In the usual conventional type of radar range unit `it is ordinarily necessaryand desirable to control the gain 'and/or the sensitivity of the radar receiver with which the range unit is associated so that the output thereof will remain substantially constant even though the incoming target and noise signals may vary appreciably in strength and/or intensity. Although a number of relatively complex systems and circuits have been designed in the past to accomplish the above mentioned desired end, considerable ditlculty has been experienced in devising a circuit which though relatively simple in design is sufhciently reliable and dependable for such a purpose.

It is a principal object of the present invention to provide a novel and improved automatic gain control and noise limiter circuit for a radar range unit, which Circuit is relatively simple in design and is adapted to properly stabilize and/or regulate the output'of an associated radar receiver despite irregularities and discontinuities in the strength and amplitude of the incoming signals.

Other objects and many of the `attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Figure 1 is a block diagram of a conventional type of radar range unit in which a preferred embodiment of the present invention is incorporated.

Figure 2 is a detailed circuit diagram of the apparatus shownin Figure 1.

Before proceeding with a more detailed description of the improved control circuits of the present invention a brief functional explanation of the same as well as the manner in which they are operatively associated with other component parts of the disclosed radar range unit will be given in order to aid in a complete understanding of the detailed description that is to follow. The radar range unit with which the subject matter of the present invention is preferably incorporated is generally capable of performing two important operations. First, it searches out a particular target that `is located in a given target area; then, it automatically tracks that target so as to continuously provide a voltage, the magnitude of which accurately describes the target range.

Until a suitable target has been encountered the search circuit which is normally energized and which is generally designated in Figure l of the drawing by reference numeral 3 together with firing circuit 3a causes the effective range, at which the coincidence circuit d is gated (i. e. conditioned for energization by incoming video signals on the input line 5) to increase and/ or progress relatively slowly and linearly outwardly with each successive synchronizing pulse on line 6 until a preselected maximum range is reached. At that instant the effective range at which the coincidence circuit is then gated abruptly returns to a preselected minimum range to repeat the outward searching process. This searching operation continues until a target is encountered. Moreover, until that time as will be described more fully hereinafter the AGC and noise limiter circuits 7 are proportionately controlled largely by the amplitude or strength of the noise signals that continuously excite the video input line.

When in the course of the above described outward Search a target is encountered, and when the coincidence circuit 4 has been properly gated by the firing circuit 3a, the resultant effect on the coincidence circuit is conducted through the amplifier circuit S to the alarm circuit 9, which in turn causes an immediate energization of the tracking and range voltage circuits liti. During the subsequent tracking operation the nature and amplitude of the desired AGC and noise limiter control voltage is substantially determined by and proportional to the amplitude of the incoming target signals on the video input line 5 in a manner which will also be described more fully hereinafter.

At the outset it is to be understood that although the subject matter of the present invention has been particularly devised for use with and in conjunction with the component parts of a conventional radar range unit, the same could also be used in connection with any other suitable electronic equipment without departing from the spirit or scope of the present invention. Moreover, in this connection since the specific structure and nature of such components of the radar range unit as the search circuit 3 and the tracking and range voltage circuits 10 may take a large variety of different forms and since the4 details thereof form no important part of the present invention -a full description of the same is omitted for the sake of simplicity.

A detailed circuit diagram of a preferred embodiment of the present invention is shown in Figure 2 of the drawing. As illustrated therein, the search circuit which is generally designated by reference numeral 3 is coupled to the grid of the triode amplifier V-l through the condenser C-l. The plate circuit of triode V-l extends from the positive volt line 14 through the resistor R-l and the tube to ground. The plate of V-1 is connected to the grid of the conventional single-swing blocking oscillator V-2 of the tiring circuit 3a through the condenser C-2. The plate circuit of the blocking oscillator extends from the positive 150 volt line ld through the primary Winding P of the coupling transformer T-l and the tube to ground. The series plate to grid feedback circuit of the oscillator V-Z includes the secondary winding S-l of transformer T-1 and the condensers C-3 and C-4. The junction of the winding S-1 and condenser C-S is preferably tied to ground. As will be more apparent hereinafter the resistor divider network which includes resistors R-Z and R-3 andv which extends between ground and the negative 250 volt line 15 continuously provides a suitable bias for the grid of V-Z.

The output circuit of the blocking oscillator V-Z includes in series the secondary winding S-Z of transformer T-l, resistor R-4, delay network 16, and resistor R-S. rlhe junction of the Winding S-2 and resistor R-S is preferably coupled to ground through condenser C-S and is also tied to the resistance network which extends from thenegative 250 volt line 15 to ground and which includes resistors R21 and R22. As shown in the drawing the suppressor grids of the coincidence tubes V-3 and V-4 are connected directly to the output circuit of the blocking oscillator preferably adjacent opposite terminals of the delay network 16. The plate circuits of the coincidence tubes V-3 and V-4 extend from the positive 300 volt line 17 successively' through resistors R-6, R-7Y and R-S respectively through resistors R-Sa and R-Sb and through the respective tubes to ground. The plates of tubes V-S and V-4 which are preferably coupled to ground by means of the condenser C-6 are also respectively `connected to the tracking circuit through condensers C-7 and C-8.

Y VThe control grids of thencoincidence tubes V-3 .and

V-4 are preferably tied together as shown and are coupled lthrough condenser C-9 to the output of the cathode follower V-5 which las will be more apparent hereinafter isv driven by the incoming target signals on the video inputA line 5. The grid resistor R-9 in the cathode fols lower circuit is preferably connected as shown in the drawing between the grid of V-S and ground. The plate circuit of V-S extends from the positive volt line 14 through the tube and cathode resistor R-10 to the negative 250 volt line 15.

As is also shown in the drawing the D.v C. restorer triode or clamp V-6 is preferably coupled to the input control grid circuits of the coincidence tubes 'V-3Jand V4 such that as will be more apparent thereinafter negative surges on the input line are'substantially eliminated. Accordingly, the cathode of V-6 is tied directly the coincidence tube grid input circuit whereas" its grid and plate which are preferably connected together are connected to a resistor network which extendsbetween the negative 250 volt line 15 and ground and'which includes in series resis'tors'R-ll and R-12. The cathode of V-6 is also preferably coupled to its grid andplate through resistor R-13.

The screen grid circuits of coincidence tubes V-3 and V-4 in addition to beingrespectively coupled to ground by Way of the by-pass condensers C-10 and C-11 are preferably connected to opposite extremities of the pofollower V-7 which is also preferablyconnected to -ground by way of condenser C-12.

The plate circuit of V-7 extends from the positive l5() volt line 14 through the tube and cathode resistor R-19 tothe negative 250 volt line 15. The output 'circuit' of cathode follower V-7 preferably extends through resistor R- to a suitable radarv receiver or other associated equipment which thoughy not shown on the drawing is to be controlled in yaccordance .with the present `inven tion. As shown in the drawing the above described output circuit is also preferably tied to ground by the diode clamp or the like V-8a. i

The output circuits of the coincidence tubes V-3 and V-4 are connected to the grid of the directy coupled ampliier V8 so as to drive the same in accordance with variations in voltage potentialat theplates of V-3 and V-4. The plate circuit of'V-S extends from the positive 300 volt line 17 throughY resistor R-6 and the tube to the positive 150 volt line 14. v As shown in the drawingv the plate of triode V-8 is operatively coupled to the grid of D. C. amplifier V-9by.means of the series circuit to ground, which-circuitincludes resistors R-7 and R-23 and potentiometer R-24.` The plate circuit of V-9.which extends from the -positive 300vvolt line 17 through re` sistor R-,ZS and the tube tothe positive 150 volt line 14, drives the series` parallelresistance network which inand V-4 the screen circuits thereof continuously follow Y reduced. This abrupt negatively-directed excursion i energization of the screen circuits thereof, and according'l on the drawing.

4 'clud'es resistors R-26, R-27, R28, and R'-29 and whicift"` is connected between the plate of V-9 and the negativev 250 volt line 1S. The junction of resistors R- 28 R-29 of this network is coupled to the grid of cathodevv follower V-7 through'the diode V-10 whereasthe junoV tion of resistors R-26 and R-Z7 is coupledto the of the alarm relay triode V-11. The grid of V-11 is also Y. coupled to ground by way of condenser C-13 whereas its plate circuit extends from the positive l5 0,volt li 14 through relay winding K and the tube to ground.

- In operation the incoming video target signals togethe withbackground noise signals and the like are receiv on the input line 5 and passed through the isolating cath i ode follower V-.5 to the control grid circuit of the coincidence tubes 'V-a-Yand` V-4. Inasmuch aslthe resis ance of resistor R 11 in the Video input clamping circuit. is relatively large compared to that of resistor R-12the" grid and plate of triode V-6 is' maintained at a negativ potential which is preferably ronly slightly below groun levelsuch that the negative surges on the input line ar substantially eliminated; l j j A During the searching operation, since no target sign is delivered to either control grid of the coincidence tubes V-3 and`V-4 while theV suppressor grids'thereof areposi tivelyfpulsed by the blocking oscillator VK-2,j th tube( V-3 and V-4 remain cut off so far as their plate circniY L are concerned. However, despite. the normal relatively. high negative bias on the suppressor grids. of tubes kV-S the variations of voltage potential onthevideo input S. More specifically the .noise signals as occasional incoming target signals excite the grid circm of eachfof the coincidence tubes, produce a proportiona drive the grid circuit ofthe cathode follower VV-7, which as will bernore apparent hereinafter provides the desired: AGCl and noisev limiter control voltage for a suitable: radar receiver or other associatedequpment not sho'` When duringfthe searching operation atarget'is'vul mately encountered'the plate circuits ofthe coinciden tubes V-3 and V-4 become energized such thatf'tle tential at the plates thereof is abruptly and substantial] the plates of the coincidence tubes `V3 `a1' d,V-4 is't successively passed' through the D. C. amplifier ci'rcl ,f whichrinciudes modes v-s andy-91eme associatedseries parallel resistancenetwo'rk, to which a pair af-"ol J put circuits are coupled.l One of these outpu't`ci`rcj'1ik extends from the junction of the network `resistors and R 27 ythrough resistor R-30" to 'the grid of .tried V-11 and thereby controls the 'nergization of alarm rl K in the plate circuit of 'V-11. When relayfK Y energized in this way the tracking and range voltage 'cir'y cuits 10 are then immediately made operative' Y Y The other above mentioned outputcircuitof the'said resistance networkextends fromfthe junction'of its com ponent resistors R-28 and R-29 throughdiode` V`10 the grid of cathode follower V-7 where the AGC: signal;k and noise limiter signal from the coincidencetube' "screen" circuit'are combined to provide the desired 'sensitivity'. control voltage on the output line 18 forthe receivero other suitable associated radar equipment. y I K Obviously many modifications andvariations of the' present invention are possible in the light ofthefabov teachings. It is therefore to be understood-that'iwitliil'll the-scope ofthe appended claims the invention maybi practiced otherwise vthan as specically describedw what is claimed isf Y 1. In a radar lrangeunit a source of video' target 'si nais-superimposed upon a background' of noise; a `ga. ing circuit including a pair of coincidence tubes, ea'chofL the said tubes having a common control grid yenergizi circuit,"a'common screen grid energizing circuit, a` com mon suppressor grid energizing circuit, and a'com'moilplate energizing circuit; means for normally maintaining a substantial negative bias upon the suppressor grid circuit and for intermittently removing the said negative bias therefrom; means for normally biasing the control grid circuit to cut-off; means for coupling the control grid circuit to the said source; a cathode follower circuit; means for driving the cathode follower circuit in accordance with the ow of current through the screen circuit; means for also driving the cathode follower circuit in accordance with the flow of current through the plate circuit; and means for obtaining an automatic gain control and noise limiter voltage from the output of the cathode follower circuit.

2. In a radar range unit a source of video target signals superimposed upon a background of noise; a gating circuit including a pair of coincidence tubes, each of the said tubes having a common control grid energizing circuit, a common screen grid energizing circuit, a common suppressor grid energizing circuit, and a common plate energizing circuit; means for normally maintaining a substantial negative bias upon the suppressor grid circuit and for intermittently removing the said negative bias therefrom; means for normally biasing the control grid circuit to cut-olf; means for coupling the control grid circuit to the said source; an amplifier; means for driving the amplifier in accordance with the ilow of current through the said plate circuit; a cathode follower; means for coupling the said screen circuit to the input of the cathode follower; a diode; means for coupling the output of the amplifier through the diode `also to the input of the cathode follower; and means for obtaining an automatic gain control and noise limiter voltage from the output of the cathode follower.

References Cited in the le of this patent UNITED STATES PATENTS 2,241,581 Boucke May 13, 1941 2,451,632 Oliver Oct. 19, 1948 2,516,356 Tull et al. July 25, 1950 2,538,028 Mozley Jan. 16, 1951 2,563,902 Yost Aug. 14, 1951 2,677,050 Crooks Apr. 27, 1954 2,734,134 Beard Feb. 7, 1956 

